EFFECTS OF TEMPERATURE AND OXYGEN CONCENTRATION ON
TORREFACTION OF OIL PALM KERNEL SHELL
Shazleen Saadon1a, *Yoshimitsu Uemura1b, and Nurlidia Mansor1c
Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia.
1a
shazleen.saadon@gmail.com; 1b*yoshimitsu_uemura@petronas.com.my;
1c
nurlidia_mansor@petronas.com.my
1
Abstract
Torrefaction of palm kernel shell (PKS) has been studied to determine the solid yield
after torrefaction. The temperature was varied from 220 to 300 ˚C and the reaction time was
studied between 30 and 90 min. Temperature influenced the solid yield significantly, in which
the resulting solid yield was from 93 to 59 % along the increment of the temperature.
Reaction time had only small influence on the solid yield. For both process parameters,
torrefaction experiments in the presence of 3, 9 and 15 vol% of oxygen were compared to a
control experiment (inert torrefaction). The solid yield decreased slightly with an increase in
oxygen concentration because the biomass reacted with oxygen in oxidation reaction in
parallel with the torrefaction reaction.
Keyword: Torrefaction, palm kernel shell, temperature, oxygen, solid yield
Introduction
The abundance of lignocellulosic biomass in Malaysia such as oil palm wastes motivates
a potential for energy conversion. The oil palm wastes include empty fruit bunch, palm
mesocarp fiber and palm kernel shell that make up to approximately 70,000 million tons
generation each year. The biomass can be converted into energy via gasification, pyrolysis,
combustion and many other process routes. The utilization of biomass for energy production
can help to reduce the dependency on fossil fuels thus ensures the sustainability of the energy
supply. However, direct utilization of lignocellulosic biomass entails several shortcomings
compared to fossil fuels; such as high moisture content, low calorific value and low bulk
density. Because of these poor characteristics, producing energy will need a massive amount
of biomass to compete with the energy produced by the fossil fuels, adding to the cost of
transportation and storage. This fact has made clear that biomass cannot simply replace the
existing fuels (coal and oil) without a pre-treatment.
A pre-treatment for biomass known as torrefaction can help to improve these inferior
properties by decomposition of carbohydrate chemicals in the biomass such as hemicellulose
and parts of cellulose and lignin. It involves slow heating of biomass in an inert atmosphere to
a temperature not more than 300 °C. The main product of torrefaction is solid char with a
reduced moisture content and increased calorific value as a result of removal of bound water,
fractions of organics and ash, hence increasing the energy density of the torrefied biomass.
By-products of torrefaction are condensable liquid that consists of water, organic and inorganic
solubles; and non-condensable gases that consist of carbon dioxide and carbon monoxide
mostly.
The torrefied biomass can be used as a prospective feedstock for many industrial
applications, for instance co-firing with coal in power generation plants and in entrained-flow
gasifier. In co-firing with coal, the power needed in sizing the torrefied biomass is reduced up
to 70-90% compared to untreated biomass due to its improved grindability. This characteristic
enables the torrefied biomass and coal to be mixed together in the grinder during the sizing
process because of equal grindability between coal and torrefied biomass, hence reduced the
feed handling cost. As in entrained-flow gasifier, the pre-treated biomass often facilitates the
conversion into biosyngas.
Product distributions, characterization and the suitability of various feedstocks for
torrefaction have been reported by many researchers. Different parameters have been
exercised to study its influence on the properties of the torrefied product. Medic et al. [1]
concluded that biomass with high moisture content loses most of its mass due to the
expansion of water vapor that enhance the heat transfer in the biomass sample. The effect of
temperature on torrefaction of biomass usually is a main parameter investigated because the
severity of thermal energy supplied to the biomass affected the decomposition of
hemicellulose, cellulose and lignin to some extent. The higher the temperature, the solid yield
decreased from approximately 97 to 50 %, depending on the type of biomass because
different biomass has different hemicellulose content [2-5]. Arias et al. [6] reported that
torrefaction increased the brittleness of the biomass hence the grindability was improved.
During the heat treatment, the fibrous nature of the biomass that resistant to grinding was
gradually extinguished, making the energy consumption for grinding reduced. Pimchuai et al.
[7] concluded that the heat of combustion of torrefied biomass was higher than that of raw
biomass due to the increased high heating value and fixed carbon content.
Most of the studies only reported the influence of temperature, residence time and
different feedstocks on the torrefaction performance; however the effect of atmospheric
condition is important just as much. Torrefaction normally utilizes a continuous supply of
nitrogen to create non-oxidizing atmosphere in the reactor. However, the objective of this
study is to investigate the torrefaction behavior of biomass if oxidizing atmospheres were
employed. In order to do that, oxygen with a pre-determined concentration was supplied
along with nitrogen throughout the experiment. The aim is to simulate the composition of flue
gas wherein generally contains a distributed amount of nitrogen, oxygen, carbon dioxide,
water and other gases depending on the type of reaction. Table 1 shows a summary of a
typical flue gas composition from variety of process plants. In this paper, torrefaction in the
presence of 3, 9 and 15 vol% of oxygen was studied on the basis of the data shown in the
table. From the economic point of view, the cost of nitrogen gas supply is subjected to the
volatile price of natural gas that is currently suffered from depletion. While torrefaction in the
presence of oxygen is non-generic, it is beneficial in such a way that it creates an opportunity
to utilize a flue gas from boilers as a torrefaction gas to reduce the dependency on pure
nitrogen, hence cutting off the cost of supply and maintenance in torrefaction plant.
In this paper, the effects of oxygen concentration, pyrolysis temperature and time on
torrefaction behavior of PKS were investigated to clarify how much oxygen concentration can
be tolerated for torrefaction of the biomass.
Table 1. Typical Composition of Some Raw Flue Gases And Fuel Gases Before Gas
Clean-Up
O2 %-v
N2 %-v
CO2 %-v
H2O %-v
CO %-v
H2 %-v
Ar %-v
SO2 ppmw
H2S ppmw
NOx ppmw
NH3 ppmw
HCN ppmw
HCl ppmw
HF ppmw
dioxine ppb
CH4 %-v
CnHm %-v
Hg ppmw
Cd ppmw
other heavy metal ppmw
Pulverized coal
combustion flue
gas
~6
~ 76
~ 11
~6
Waste
incineration
flue gas
7 - 14
balance
6 - 12
10 - 18
0.001 - 0.06
~1
~1
200 - 1500
500 - 800
200 - 500
Coal
gasification
fuel gas §
~4/~1
~ 4 / ~ 13
~4 / ~ 1
~ 58 / ~ 40
~30 / ~29
~1
Coal-fired
IGCC flue gas
Natural gas
Groningen
Gas-fired CC
flue gas
~ 12
~ 66
~7
~ 14
~ 14
~1
~ 14
~ 76
~3
~6
~1
10 – 200
~1
10 – 100
10 – 300
1000 - 4000
<< 1
400 - 3000
2 - 100
1 - 10
0.1 - 1
0.01 - 1
0.5 - 2
< 0.002
0.1 - 1
0.1 - 0.5
1-5
300 - 800
40 - 150
500 - 600
150 - 250
~ 81
~4
0.1 - 1
0.1 - 0.5
~ 20
Methodology
Raw material
The palm kernel shell (PKS) used in this study had a particle size between 0.5 to 1 cm,
and was collected in bulk from an oil palm mill in Perak, Malaysia. The biomass was pretreated by oven-drying at 105 ˚C for 24 h, and then kept in an airtight container to protect
from property changes. Table 2 shows the physical properties of the raw biomass.
Experimental procedure
Approximately 3 g of PKS sample was torrefied in a vertical tubular, stainless steel
reactor (internal diameter 0.028 m, length 0.56 m). The reactor was first flushed with nitrogen
gas for 15 min to drive out the existing oxygen. Then the temperature was ramped to the
desired temperature at 10 ˚C/min. The torrefaction reaction was allowed to take place for
particular desired reaction time at that temperature before the reactor was cooled down for 2
h. Finally the torrefied sample was retrieved and weighed immediately to avoid moisture gain.
Table 2. Physical Properties of Raw Material PKS
Proximate analysis [wt %]
Moisture
10.00
Ash
0.84
Elemental analysis [wt %]
C
50.62
H
6.02
N
0.37
O
42.15
HHV [MJ/kg]
20.1
Chemical composition analysis [wt %] [8]
Hemicellulose
26
Cellulose
22
Lignin
46
The product gas evolved from the process was flowed through the condenser which
consists of a collection vessel submerged in an ice trap to maintain the temperature below
5 °C. Condensable gas was collected in the collection vessel, while non-condensable gas
sample was captured every 10 min and was injected to gas chromatography with thermal
conductivity detector (GC-TCD). Throughout the process 30 ml/min of torrefaction gas was
flowed into the reactor. Torrefaction gas refers to the combination of nitrogen and oxygen
flowed into the reactor to create the desired atmospheric conditions. The effects of
temperature, reaction time and oxygen presence during torrefaction of palm kernel shell are
discussed in the next section.
Results and Discussion
Total mass balance
Total mass balance of torrefied PKS is expressed in terms of solid, liquid and gas yields
as tabulated in Table 3.
The process parameters investigated in this study are reaction time and temperature, in
which the oxygen concentration is varied in each set of process parameter. Inert torrefaction
represented by 0 vol% of oxygen sets as a control, while oxidative torrefaction is represented
by 3, 9 and 15 vol% of oxygen. Reaction time is defined as the duration of the torrefaction
reaction; which is allowed to take place once the desired temperature is achieved. 30, 60 and
90 min of reaction time were investigated in this study by reviewing the previous literatures as
reference. The torrefaction temperature was varied at 220, 250 and 300 ˚C because studies
have shown that at temperature lower than 220 ˚C the torrefaction performance is indistinct.
Solid yield represents the percentage of biomass solid retained after treated with torrefaction.
Liquid yield collected as a result of gas condensation often consists of water and acetic acid,
while carbon dioxide (CO2) and carbon monoxide (CO) made up most of the gas yield. The
CO2 was produced as a result of decarboxylation of acid groups attached to hemicellulose
component in the biomass, while CO was produced in the reaction of CO 2 and steam with char
with increasing temperature [1]. Liquid and gas yields showed insignificant trend, maybe due
to the physical or chemical inhomogeneity of the PKS particles that was being torrefied, or the
inadequacy of recovering the liquid and gas products that contribute to the fluctuating results.
Further elaboration on the effect of process parameters on the yield of solid is well-described
in the next section.
Table 3. Total Mass Balance of Torrefied Biomass
Oxygen
concentration
[vol%]
0
3
9
15
0
3
9
15
0
3
9
15
0
3
9
15
0
3
9
15
Reaction
time [min]
Temperature
[˚C]
Solid yield
[%]
Liquid
yield [%]
CO2 yield
[%]
Other
gases [%]
30
30
30
30
60
60
60
60
90
90
90
90
30
30
30
30
30
30
30
30
250
250
250
250
250
250
250
250
250
250
250
250
220
220
220
220
300
300
300
300
81.97
81.89
81.60
76.78
80.63
80.01
78.57
77.90
79.38
78.87
77.98
76.34
93.43
93.30
93.19
92.41
64.13
63.46
60.54
59.16
8.32
13.42
9.91
9.22
9.32
5.06
9.98
15.75
10.54
9.90
11.06
15.94
6.13
4.81
5.91
1.78
20.06
14.82
20.64
18.29
7.81
3.10
6.68
9.64
8.04
4.91
10.03
0.16
8.75
4.16
13.19
3.88
1.67
1.36
2.27
0.88
14.29
1.52
8.05
8.37
1.90
1.59
1.81
4.36
2.01
10.02
1.42
6.19
1.33
7.07
0
3.84
0
0.53
0
4.93
1.52
20.20
10.77
14.18
Effect of reaction time on torrefaction
Figure 1 shows the behavior of solid yield when the biomass was torrefied at three
reaction times. The temperature was kept constant at 250 ˚C while the reaction time was
varied from 30 to 90 min. From Figure 1 it is observed that as the reaction time was
prolonged, there was an insignificant decrease in solid yield of torrefied PKS, regardless of
atmospheric condition of the torrefaction. The figure shows that the solid retained is in the
range on 76 to 82 % only. This may have happened because the concentration of biomass
was the limiting factor, in which reaction was substantially terminated after 30 min of
torrefaction, thus prolonging reaction time did not decrease the solid yield any further.
Inert
with 3 vol % oxygen
with 9 vol % oxygen
with 15 vol % oxygen
100
Solid yield [%]
80
60
40
20
0
30
60
Reaction time [min]
90
Figure 1. Solid Yield of PKS as a Function of Reaction Time in Inert and Oxidative
Torrefaction.
Effect of temperature on torrefaction
Figure 2 shows the effect of temperature on solid yield of torrefied PKS. The reaction
time is kept constant at 30 min throughout these experiments based on the result obtained in
the previous section. It can be observed that when the temperature increased, the solid yield
decrease for all atmospheric condition of torrefaction. When the PKS was treated in mild
torrefaction (220 ˚C), about 93 % of the mass was retained, while PKS treated in severe
torrefaction (300 ˚C) retained about 60 % of mass. The decrease in solid yield can be
explained by the enhanced degradation of lignocellulosic compound of biomass at higher
temperatures. Hemicellulose, the most reactive component of lignocellulosic biomass,
decomposes at 220 to 250 ˚C while cellulose decomposes from 240 to 350 ˚C. Lignin, being
highly resistance to thermal degradation, decomposes only at 280 to 500 ˚C [9]. As the
torrefaction proceeded, some mass loss occurred. PKS torrefied at higher temperature caused
more extensive disintegration of the lignocellulosic components liberated into volatiles, thus
the concentration of each component decreased; explaining the attenuation of the residual.
Chemical composition analysis in Table II shows that PKS has high content lignin (46 wt%)
therefore even though at temperature as high at 300 ˚C only approximately 50 % of mass
was lost.
It was also observed that the solid yield of torrefied PKS in oxidative torrefaction was
lower than that in inert torrefaction. The difference in solid yield between the two atmospheric
conditions may be attributed to oxidation of biomass to form gaseous product in oxidizing
atmosphere. Torrefaction in the presence of oxygen caused oxidation of biomass to occur
alongside torrefaction reaction, and the increase in temperature initiated to a higher oxidation
stage [14]. Thus, more mass loss occurred in oxidative torrefaction than that in inert
torrefaction. The fact that the effect of oxygen atmosphere on the yield was more prominent
at higher temperatures may be attributed to the difference of activation energy between
oxidation and non-oxidation decomposition.
Inert
with 3 vol % oxygen
with 9 vol % oxygen
with 15 vol % oxygen
100
Solid yield [%]
80
60
40
20
0
220
250
300
Temperature [˚C]
Figure 2. Solid Yield of PKS as a Function of Temperature in Inert and Oxidative
Torrefaction.
Conclusion
Torrefaction of palm kernel shell (PKS) at different reaction times, temperatures and
atmospheric conditions was studied. Reaction time did not give significant impact as much as
temperature because the decomposition of lignocellulosic biomass was terminated within the
shortest torrefaction time applied (30 min). Lignin in PKS was highly resistant to the
temperature, so that approximately 50 % of the biomass solid was still retained after being
treated with severe torrefaction at 300 ˚C. When the concentration of oxygen was varied from
3 to 15 vol%, the solid yield was lowered by only 7 % compared to the inert torrefaction
(control). This insignificant decrease may be an indicator that torrefaction in the presence of
low percent of oxygen is rational, since the torrefaction performance is not much affected by
the existence of oxygen up to 15 vol%. The authors’ intention is to further investigate the
effect of torrefaction in the presence of carbon dioxide, so as to demonstrate if the flue gases
from boilers can be used as torrefaction gas.
Acknowledgment
This study was financially supported by the Mitsubishi Corporation Education Trust Fund.
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